Offshore installations which utilize articulated towers, can be utilized in a variety of applications. In the mooring of ships, the tower is allowed to pivot by a wide angle such as 20.degree. from the vertical, to accommodate motions of the ship, but the tower is biased towards the vertical position. In the drilling of undersea wells the tower can serve as a fixed platform, but means must be provided to prevent large pivoting of the tower away from an initial vertical position.
An articulated tower that is allowed to pivot by a wide angle, can be biased towards the vertical by the use of a buoy formed along the upper portion of the tower. An articulated tower utilized in an application where very little pivoting is allowable, can be held in its vertical orientation by taut guy wires extending in different directions from the top of the tower to the seabed. In each of these systems, the above techniques for urging the tower towards the vertical makes it "stiff" against pivoting, which can result in large stresses on the structure. The forces that tend to move the tower include long term moderate forces resulting from currents and winds, and large but short duration forces caused by waves. A tower which is stiff against pivoting will have a relatively short natural period of oscillation about its pivot point, and if this period is low enough to be close to the period of waves then the tower tends to be moved considerably by the waves. For articulated towers utilized in shallow water with large buoys, the tower tends to have a large stiffness, or short period of oscillation, that may match that of the waves, so that the tower is affected to a large extent by waves. Towers in relatively shallow water that are held against large pivoting by taut guy wires, also have a high stiffness that produces a relatively short period of natural oscillation. Offshore articulated tower systems which could be biased towards their vertical orientation in a manner that minimized the stiffness of tower pivoting to maintain a relatively long natural period of oscillation, would enable articulated towers to be utilized in relatively shallow water without subjecting them to high forces from waves.